1. Field of the Invention
The present invention relates to a mixed-flow compressor, and more particularly to a mixed-flow compressor suitable for enhancement of its performance and for compactness in size.
2. Description of the Prior Art
As disclosed in Proceedings of the Sixth Turbomachinery Symposium, pp 61 to 62 (October 1977), in a conventional mixed-flow compressor, an oblique or mixed flow diffuser (in which a flow-out direction is leaned from the radial direction) is not provided with guide vanes in the flow path. Also, if provided as disclosed in Japanese Utility Model Examined Publication No. 56-38240, the guide vanes are arranged on a core plate side. In addition, it is also known such as in Journal of the Japan Society of Mechanical Engineers, pp 16 to 20 (March 1987) that a diffuser can be provided in the radial direction, however, guide vanes are not provided.
In general, if the specific velocity ns (expressed in the following equation) is high, an impeller inlet tip to outlet tip diameter ratio is increased, so that the performance of the impeller becomes low because the curvature of meridional flow passage is increased in the case of a centrifugal impeller. Further, because of an increase in curvature, a secondary flow becomes remarkable within the impeller, so that the flow at the outlet of the impeller is deflected to the hub side to thereby lower the performance of the diffuser. The specific speed ns is defined as: ##EQU1## where N is the rotational speed (rpm), Q is the volume flow rate (m.sup.3 /min) and Had is the adiabatic head (m).
In order to avoid this problem, in general, a mixed-flow impeller in which the outlet of the impeller is leaned from the radial direction is used. In the mixed-flow impeller, the curvature of a meridional flow passage in a meridional plane (i.e., a cross section including the rotary shaft center, and (hereinafter referred simply as a meridional flow passage) is decreased, so that the flow may be kept substantially uniform in the widthwise direction at the outlet of the impeller, i.e., at the inlet of the diffuser. It is thus possible to prevent the generation of the flow deflected toward the hub side. However, if the flow having a volute component enters into the mixed-flow diffuser, the flow is deflected to the shroud side from the intermediate portion to the outlet portion of the diffuser due to the curvature which is perpendicular to the flow passage. In the extreme case, a reverse flow will be generated on the hub side to largely increase the diffuser pressure loss. In addition, in the mixed-flow compressor in which such a mixed flow diffuser without vanes is used, the length in the axial direction is increased to make the compressor large in size and to increase the friction loss in the flow passage.
Furthermore, since the length of the rotary shaft is increased, the critical speed of the rotor must be lowered.
In order to overcome these problems, guide vanes each having a height (for example, the projecting distance from a plate on a shroud side into the diffuser flow passage in a direction substantially transverse to the direction of flow) corresponding to a range from 10 to 50% of the width of the meridional flow passage and are provided on the hub side of the diffuser. However, this could no sufficiently attain the object. There are still unsolved problems such as the increase in friction loss and a reduction in critical speed.
In order to overcome these problems, an object of the present invention is to provide a mixed-flow compressor that is small in size and ensures a high performance.